In many applications of single-mode optical waveguides, such as sensors and coherent optical communication systems, it is important that the propagating optical signal retain the polarization characteristics of the input light in the presence of external or internal depolarizing perturbations. The change of the polarization state of a propagating signal can be prevented or reduced by employing a fiber that is birefringent, i.e. the core refractive index is different for two orthogonally polarized light waves.
The polarization performance of a single-mode fiber can be characterized by its beat length L, where L is defined as 2.pi./.DELTA..beta., and .DELTA..beta. is the difference in the propagation constants of the two orthogonal polarizations. It is desirable to make fibers with a beat length L of 1 mm or less.
Optical fibers in which a slight improvement in polarization performance is achieved by distorting the core symmetry are disclosed in U.S. Pat. No. 4,184,859 and in the publication by V. Ramaswamy et al., "Influence of Noncircular Core on the Polarization Performance of Single Mode Fibers", Electronics Letters, Vol. 14, No. 5, pp. 143-144, 1978. However, the Ramaswamy publication reports that measurements on borosilicate fibers with noncircular cores indicate that the noncircular geometry and the associated stress-induced birefringence decrease L to about 5.5 cm and hence are not sufficient to improve polarization performance in single-mode fibers to the extent necessary to achieve polarization stabilized, single-mode propagation.
The inventions disclosed in U.S. Pat. Nos. 4,179,189 and 4,274,854 are based upon the recognition that orthogonally polarized waves are more efficiently decoupled in a waveguide that is fabricated in such a manner as to deliberately enhance stress-induced, or strain birefringence. Those patents teach that such behavior is accomplished by introducing a geometrical and material asymmetry in the preform from which the optical fiber is drawn. The strain-induced birefringence is introduced by at least partially surrounding the single-mode waveguide by an outer jacket having a different thermal coefficient of expansion (TCE) than that of the waveguide and a thickness along one direction that is different from its thickness along a direction orthogonal to the one direction. Other fibers having stress-induced birefringence are taught in U.S. Pat. Nos. 4,354,736 and 4,360,371.
The publication, H. Matsumura et al., "Fundamental Study of Single Polarization Fibers", Proc. Sixth European Conference on Optical Communication, University of York, U.K., September 1980, pp 49-52, describes a fiber of the type disclosed in the aforementioned U.S. Pat. No. 4,360,371. Such a fiber exhibits a low beat length L because of both an anisotropic stress and a non-circular core. A minimum beat length of 0.77 mm is reported for a fiber having a core of silica doped with 27 mole % GeO.sub.2, the core exhibiting an elipticity of greater than 80%. However, such a fiber would possess an attenuation of approximately 20 dB/km or more, thus making it unsuitable for use as a long distance transmission line. This publication reports that a B.sub.2 O.sub.3 -doped single polarization fiber suitable for use as a long distance transmission line exhibited a coupling length L of 1.5 mm.
A single-mode fiber having an azimuthally asymmetric refractive index profile to increase the difference between the propagation constants of the two orthogonal polarizations of the single mode signal is taught in the publication, T. Okoshi et al., "Single-Polarization Single-Mode Optical Fibre with Refractive-Index Pits on Both Sides of Core", Electronics Letters, Vol. 16, No. 18, Aug. 28, 1980, pp 712-713. This publication describes a fiber having a circular central region which is divided into a central, rectangularly-shaped core and circular segments of low index along the long sides of the central region. That fiber is reported to have a beat length of 2.3 mm.
The stabilization of the polarization state in an optical waveguide has also been accomplished by employing an optically anisotropic single crystal as the light conducting medium. For example, see U.S. Pat. No. 4,077,699 (Dyott et al.). That the birefringent properties of crystals may be explained in terms of the anisotropic electrical properties of the molecules of which the crystals are composed is pointed out in the text: Born et al. Principles of Optics, Pergamon Press, New York (1975) pp 705-708. It is further stated that "form birefringence" may also arise on a scale that is larger than molecular when there is an ordered arrangement of similar particles of optically isotropic material whose size is larger compared with the dimensions of molecules but small compared with the wavelength of light. Such a medium may be visualized as an assembly of thin parallel plates of thickness t.sub.1, dielectric constant .epsilon..sub.1 and refractive index n.sub.1 immersed in a medium having a dielectric constant .epsilon..sub.2 and refractive index n.sub.2. The widths of the spaces between the plates is t.sub.2. After deriving the difference between dielectric constants parallel to the plates and normal to the plates, Born et al. conclude that the composite assembly behaves like a negative uniaxial crystal. The difference between the refractive indices n and n parallel to and normal to the plane of the plates, respectively, is given as ##EQU1## where f.sub.1,=t.sub.1 /(t.sub.1 +t.sub.2) and f.sub.2 =1-f.sub.1.
The aforementioned Dyott et al. patent indiates that an optical waveguide fiber having a crystalline core and glass cladding is subject to such defects as voids lying between the crystal and the glass. Also, since the cylindrical core must be formed as a single crystal, the formation of long lengths of such fiber is very difficult, 200 mm lengths of defect-free crystal being mentioned in said Dyott et al. patent.